Thousands of protesters surrounded the White House Sunday to let President Obama know they wouldn’t take anything but “no” for an answer when it comes to a large oil pipeline in the works.

Organizers estimated that 10,000 to 12,000 protesters, a mix of environmentalists and members of Occupy Wall Street, gathered Sunday afternoon to ring the White House and demand President Obama reject the proposed Keystone XL pipeline.

The Keystone XL pipeline would stretch from the Athabasca Tar Sands in Canada all the way to refineries along the Gulf of Mexico, passing through six states along the way.

Environmental organizations are aghast at the plan, which they say would be disastrous to fragile ecosystems in the pipeline’s path, as well as contribute to global warming.

TransCanada — the corporation proposing the $7 billion project — said it will help cut U.S. dependence on foreign oil and create more than 20,000 jobs.

The State Department was supposed to make a decision on the pipeline earlier this year, but it signaled last week that it might delay the decision for further review.

Environmentalists have made it clear that they would view it as a grave betrayal if Obama approves the pipeline. The president already angered activists when he hired a former Keystone XL lobbyist, Broderick Johnson, to be a senior staffer on his re-election campaign.

“It stinks,” said Bill McKibben of TarSandsAction.org, which has been leading protests against the pipeline. “I don’t think you could conceive a more elaborate way to disrespect not just the environmental community but also Occupy Wall Street, because this is simply a reminder of the way that corporate lobbyists dominate our politics. Forget ‘Hope and Change’ –- it’s like they want their new slogan to be ‘Business as Usual.’”

“Stop the pipeline. Yes we can,” the protesters chanted Sunday in a not-so-subtle play on Obama’s 2008 campaign slogan.

Opposition to the pipeline has garnered support from big environmental organizations like the Natural Resources Defense Council and the Sierra Club, as well as Hollywood celebrities and high-profile liberal figures.

Liberal author Naomi Klein spoke at Sunday’s protest, explicitly tying the anti-corporate message of Occupy Wall Street to the environmental movement. And indeed, there was a heavy Occupy DC presence at Sunday’s protests, with chants of “we are the 99 percent” breaking out occasionally.

If the environmental movement and the Occupy Wall Street movement coalesce in anger over what they see as broken promises and corporate cronyism in the Obama administration, it could spell trouble for the president, who needs to hold together his base in order to recreate his 2008 election success.

The Obama administration is still mulling over the decision, but recent comments by the president indicate he is cautious about the project.

“Folks in Nebraska, like all across the country, aren’t going to say to themselves, ‘We’ll take a few thousand jobs if it means our kids are potentially drinking water that would damage their health,’” Obama said in an interview with an Omaha TV station. “We don’t want, for example, aquifers to be adversely affected. Folks in Nebraska obviously would be directly impacted.”

Canada has been heavily lobbying the U.S. to approve the project. Canadian Prime Minister Stephen Harper, who has called the pipeline a “complete no-brainer,” said he wasn’t worried about Obama’s statements.

“I read the president’s comments,” Harper told reporters Friday. “I thought on balance they were non-committal and he indicated he had yet to make a decision. We respect that.”

Anti-pipeline protesters targeted Obama earlier this summer with rounds of sit-ins on the sidewalk outside of the White House. The sit-ins led to the arrests of more than 1,000 people, including James Hansen, the head of NASA’s Goddard Institute for Space Studies, and actress Daryl Hannah.